In the design of integrated circuits, designers utilize libraries of pre-designed versions of commonly used electrical circuits, elements, or blocks. Such libraries are known as standard cell libraries and contain various standard cells—including standard circuits or blocks such as flip-flops, inverters, NAND gates, XOR gates, adders, etc. For instance in a digital design, a library may contain a standard cell design for an XOR gate which has, as part of the standard cell design, the underlying transistors of the XOR gate already interconnected. In this manner, a designer merely selects the standard cell of an XOR gate to use in the design, without having to spend time designing the XOR gate at the transistor level.
In the design of an overall integrated circuit, a large number of various standard cells are connected together. A typical standard cell library may have 200 or so different types of standard cells, and a circuit design may involve numerous standard cells, such as 30,000 cells or even 1,000,000 or more cells.
As recognized by the present inventors, one problem in the design of integrated circuits, particularly complex integrated circuits, is that once the standard cells for a circuit design have been selected, the designer may wish to adjust the electrical characteristics of the circuit design.
Accordingly, as recognized by the present inventors, what is needed is a method for designing an integrated circuit which permits systematic substitution of standard cells with substitute cells having differing electrical characteristics, preferably without adversely affecting the speed characteristics of the resulting integrated circuit design.
It is against this background that various embodiments of the present invention were developed.